Disc drives employ a variety of means for moving a head/arm assembly across a magnetic medium. Demands for increased performance and capacity have lead to the nearly exclusive use of rotary voice coil motor ("VCM") actuators as the motive force for moving the head/arm assembly.
A rotary VCM typically includes a coil of wire positioned between opposing fixed magnetic structures. The fixed magnetic structures include one or more magnets connected to plates (also referred to as "poles" by those skilled in the art) which are fabricated from magnetically permeable material such as steel. The wire coil is shaped to include two opposing radial arms (FIG. 1) which allow direct current to pass in opposite directions through the radial arms. A magnetic field generated by the direct current passing through each radial arm interacts with the permanent magnetic field to apply a motive force to the radial arm. However, movement of the coil between the magnetic structures is limited so that each opposing arm experiences an opposite magnetic flux (FIG. 2). The current flowing in opposite directions through the two arms cooperates with the opposite magnetic flux experienced by each radial arm to ensure that the motive force applied to the two radial arms is cumulative.
Sophisticated control logic applies a precise amount and polarity of direct current to the windings within the coil to controllably move the coil within the fixed magnetic field. The coil and the head/arm assembly of the disc drive are attached on opposite sides of a pivot shaft (FIG. 1) to move the head/arm assembly across the magnetic medium in response to movement of the coil. The speed with which the coil moves between the fixed magnetic structures of the VCM, and thus the speed or "access time" of the disc drive, depends on the torque capability of the VCM. This torque capability depends in turn on a number of factors including the size and strength of the permanent magnet, the number of windings contained within the coil, the amount of power applied to the coil and the size of the plates above and below the coil, among others.
FIG. 2 illustrates a schematic of a prior art VCM comprising a top plate 20, a magnet 22, a coil 24 (shown in section) and a bottom plate 26. The magnet 22 is divided along a centerline 27 between North and South poles 28 and 30, respectively, and flux lines 32 illustrate the magnetic circuit between the poles of the magnet 22 and the plates 20 and 26. An air gap 34 between the magnet 22 and the bottom plate 26 allows for movement of the coil 24 within the fixed magnetic field. The total height dimension or vertical space allotted for the VCM (i.e., the distance between the top of the top plate 20 and the bottom of the bottom plate 26) is commonly referred to as the "z-height" of the VCM.
The desire to increase the access time for disc drives conflicts with a further desire to reduce the size of such drives. Indeed, relatively small hard drives (e.g., drives approximately 1 cm high) are highly desirable for use with notebook or smaller-sized computers. Because the VCM typically fits within an outer casing of the disc drive, the z-height for the VCM of a small disc drive will be less than 1 cm. Thus, a number of compromises are typically necessary in the design of a VCM. For example, in order to increase the size (and thus the power) of both the magnet 22 and the coil 24, the thickness of the top and bottom plates 20 and 26 may be reduced to the point where magnetic flux leaks outside of the plates 20 and 26, as designated by the arrows 36 in FIG. 2.
Flux leakage 36 occurs when the flux density within the plates 20 and 26 exceeds the maximum flux density for the particular material (e.g., a flux density of 18,000 Gauss for steel). Flux leakage 36 is highly undesirable since the leakage of the magnetic flux outside of the closed VCM circuit reduces the power of the VCM (thereby increasing the access time) while simultaneously interfering with the electronic circuitry and the magnetic medium within the disc drive. Because the magnetic flux within a VCM depends on the strength of the magnet 22, the flux density within the plates 20 and 26 increases as the thickness of the plates decreases. Thus, FIG. 2 illustrates the case where the thickness of the plates 20 and 26 is too small to handle the flux within the VCM.
The flux leakage can further be illustrated by plotting the flux density over the length or angle of the plates 20 and 26. Due to the rotary nature of the VCM, both the plates 20, 26 and the magnet 22 are curved to follow the arcuate path of the coil 24. FIG. 3 illustrates a prior art top plate 20 and magnet 22 and further defines an angle .THETA. determined by the arc of the magnet 22. FIG. 4 illustrates the linear increase of the flux density within the plate 20 from a point adjacent each end of the plate 20 to the middle of the plate (i.e., from .+-..THETA./2 to a 0.degree. angle as shown in FIGS. 3 and 4). FIG. 4 further illustrates that the peak flux at the middle of the plate 20 exceeds the maximum flux density that can be accommodated by the steel plate 20 as designated by the horizontal line 38. Thus, FIG. 4 graphically illustrates the flux leakage 36 shown schematically in FIG. 2.
One method of reducing flux leakage is to simply increase the cross-sectional area of the plates 20 and 26. Due to the cramped conditions within the disc drive and other constraints on the shape of the plates, the cross-sectional area of the plates 20 and 26 is typically increased by increasing the height of the plates. For example, the height of the plates 20 and 26 in FIG. 2 could be increased until the peak flux density in FIG. 4 was lower than the maximum flux density of the steel plates 20 and 26 (i.e., below the dashed line 38). However, as noted above, all the components of a VCM must fit within the maximum z-height allotted within the disc drive. Therefore, increasing the height of the plates 20 and 26 to reduce flux leakage necessitates a corresponding reduction in the size of the magnet 22, the coil 24, or both, with a resulting reduction in the power of the VCM. This tradeoff between the size of the different components within a VCM requires a careful optimization process to maximize the power of the VCM for a given z-height.
FIGS. 5 and 6 illustrate one prior art method of increasing VCM power by increasing the available z-height of the VCM. In essence, an opening 40 is formed in a top cover 42 of a prior art disc drive to allow the entire top plate 20 to protrude upward through the opening 40. The z-height of the VCM is thus increased by the thickness of the top cover 20, and a label or other adhesive covering (not shown) is then placed over the opening 40 to maintain the airtight seal within the disc drive. Although the top cover 42 is only approximately 0.5 millimeters thick, this small thickness can account for approximately a 7% increase in the effective z-height for small disc drives such as a 9.5 millimeter drive which has a nominal VCM z-height of 7.2 millimeters.
Because the opening 40 must be sufficiently large to accommodate the entire top plate 20, the size of the opening 40 makes it difficult to maintain an airtight seal within the disc drive. First, due to the tight fit of the top plate 20 within the disc drive, at least one edge 44 of the opening 40 is formed immediately adjacent an edge 46 of the top cover 42. The small surface area along the edge 46 reduces the structural integrity of the top cover 42 and can lead to deformation of the top cover 42, particularly in light of the pressure applied to the top cover 42 by a gasket 48 (FIG. 6) which fits between the top cover 42 and a base 50 of the disc drive to hermetically seal the disc drive. Similar structural weaknesses may be experienced along a second edge 52 and an intermediate comer 54 of the top cover 42 depending on the shape of the opening 40.
A further problem relates to sealing the opening 40 to maintain the hermetic seal within the disc drive. In essence, the surface area between the opening 40 and the edges 46 and 52 of the top cover 42 often proves inadequate to hold an edge of a sticker or other adhesive covering (not shown). Of course, should such an adhesive covering pull away from the edges 46 and 52 of the top cover, the interior of the disc drive would be exposed to harmful contaminants such as dust or smoke that can cause a disc crash. Thus, the prior art solution of extending the entire VCM top plate 20 through an opening 40 in the top cover 42 leads to problems with both the structural integrity of the top cover and the ability to maintain a hermetic seal within the disc drive.
It is with respect to these and other background considerations, limitations and problems that the present invention has evolved.